1/3 Octave filter banks
OCTAVE ANALYSIS & SOUND LEVEL METER (SLM)
Acoustics measurements are performed for a variety of reasons, including: product design, production testing, machine performance, and process control. Crystal Instruments’ CoCo and Spider series have capable acoustic measurement facilities including real-time octave, 1/3 octave filters, and sound level meter functions. CI provides an easy to use yet powerful toolbox for acquiring and viewing acoustic signals. Digital octave band filters and raw time data recording can be performed simultaneously for detailed investigation of noise problems.
The CoCo series hardware is ideal for portable applications that require the acoustic measurement 2 to 16 channels. The Spider series meets the requirements from 4 input channels going up to 512 channels!
Onboard IEPE (ICP®) transducer power capability allows for direct connection to pre-polarized microphones when used with an ICP microphone preamplifier. Traditional condenser microphones are also easily accommodated by connecting the direct voltage signal from the microphone power supply into an input channel. White and pink noise signals can be produced using the waveform generator. This feature is very useful when performing absorption measurements using a speaker.
Real-time Octave Analysis
The Acoustic Data Acquisition software option for CoCo and Spider hardware includes Real-time Octave Filters, Sound Level Meters, and Microphone Calibration functions. These three operations allow users to perform many acoustic measurement operations.
The Octave Analysis option applies a real-time filter bank in 1/1, 1/3rd, 1/6th, or 1/12th octaves. The input time stream is split into fractional frequency-band signals (octave bands) which can be saved. Frequency weighting can be applied to the octave bands to simulate human hearing, and time weighting can be applied to adjust sensitivity to short duration events. The resulting octave spectra can be saved periodically and displayed on a waterfall plot to observe how the spectrum changes in time. The RMS time history can also be saved as a time trace of a given octave band.
The 1/1 and 1/3 octave analysis is implemented using a real-time band-pass filtering with decimation technique. The data stream is processed continuously, and fed into a bank of decimation filters. Band-pass filters are then applied to the output of each stage of the decimation filters. This provides extremely accurate filter shapes that comply with worldwide acoustic standards: ANSI std. S1.11:2004, Order 3 Type 1-D and IEC 61260-1995
Acoustic Measurement: Sound Level Meter
The Sound Level Meter (SLM) is a related application in the acoustic data acquisition software. This module is also referred to as an Overall Level Meter. The SLM applies a frequency weighting filter to the input signal and time weighting to the filter’s output. Various acoustic measurements are then extracted from both the input and output signals of this frequency weighting filter.
All of the features that you would expect from an acoustic measurement device are present…and then some! A, B, C, and Linear weighting functions; Fast, Slow, Impulse, and Peak detectors; and user selectable high and low-pass filtering. The tremendous dynamic range that all CI instruments offer take the worry out of setting voltage ranges precisely to avoid under-range or overload conditions.
Built-in Microphone Calibration
Microphone calibration is easily handled by using a traditional microphone calibrator together with the online calibration feature. Simply define the frequency and amplitude of the reference signal, and the CI system will automatically detect the input channel that the calibration signal is applied to and then calculate the necessary calibration constants. Offsets are calculated and stored for later reference. Calibration can also be accomplished by entering microphone sensitivity values directly in the channel parameter setup page.
Simultaneous Recording and Octave Analysis
Both the CoCo and Spider are designed with simultaneous time-streaming recording capability. While the acoustic analysis is processed in real time, the raw time data of CoCo can be recorded into either internal flash memory or SD card, and that of Spider can be recorded into internal flash memory or external dedicated Spider-NAS hardware.
The raw time data of all input channels can be recorded at full analysis frequency band. After recording, the Saved files can be processed by using Post Analyzer which provides the identical analysis algorithm to those available in the real time mode.